Heat treating of metal is a commonly used technique to improve material characteristics of a workpiece for specific applications. For example, surface hardening involving a change in the composition of the outer layer of an iron-base alloy through application of an appropriate thermal treatment. Typical processes are carburizing carbo-nitriding and nitriding. Application of such processes enhances wear resistance, corrosion resistance, and fatigue strength of such treated workpieces. Other heat treatment processes involve annealing and aging.
However, in order to reproducibly obtain predetermined results using these surface hardening processes control of operating parameters such as composition of the atmosphere, temperature, and pressure during the hardening process is required. This is particularly necessary for nitriding processes. From the control point of view nitriding is a very complex process influenced by thermodynamic relations at the gas/metal interface during breakup of the atmosphere's components. The exact nature of the reactions taking place, i.e. mass transport of the gaseous phase, adsorption, diffusion and nitride phase formation is determined by the kinetics of this process. In order to control this process accurate provision of the atmosphere's components as well as temperature and pressure are essential.
Normally, a heat treating process of a workpiece comprises a number of processing steps such as preheating, carburizing or nitriding, and cooling or quenching. Numerous prior art systems have been disclosed teaching cascading of various chambers for preheating, thermal treating and cooling in order to avoid, for example, cooling of the nitriding furnace for loading and unloading of a batch of workpieces. Such systems are disclosed, for example, in U.S. Pat. No. 3,598,381 issued to Schwalm et al. in Aug. 10, 1971, U.S. Pat. No. 3,662,996 issued to Schwalm et al. in May 16, 1972, U.S. Pat. No. 4,653,732 issued to Wunning et al. in Mar. 31, 1987, U.S. Pat. No. 4,763,880 issued to Smith et al. in Aug. 16, 1988, and U.S. Pat. No. 5,052,923 issued to Peter et al. in Oct. 1, 1991, which are incorporated hereby for reference.
However, these systems are very inefficient for modern applications. Nowadays, use of thermal processing of metal workpieces in order to improve their material characteristics is numerous. This results in an increasing demand of a plurality of differently treated workpieces meeting different material characteristic requirements. The above mentioned heat treating systems only allow treatment of workpieces using a same process. Furthermore, change of thermochemical processing parameters such as atmosphere composition or temperature for different workpieces requires change of the operating parameters of the heat treating cell of the system. Therefore, a complex heat treating cell being able to provide numerous different heat treating parameters is required. Additionally, change of the heat treating parameters requires a substantial amount of time for adjusting the heat treating cell, which is not acceptable in modern manufacturing processes. Another disadvantage of these prior art systems is the inefficient use of the various system components through the cascading of these components. For example, the thermochemical processing step requires substantially more time than the cooling or quenching step. Thus, during the thermochemical processing step the cooling or quenching cell is sitting idle.
However, it would be advantageous for modern manufacturing applications to divide the thermal process into steps performed under substantially fixed conditions or performed within a narrow range of conditions based on the different processing steps required for the different heat treating of workpieces. Manufacturing and operating costs would be substantially reduced if each of the processing modules is operated at substantially fixed parameters such as atmosphere composition and temperature.
It is, therefore, an object of the invention to provide a method for thermal processing workpieces by dividing the thermal process into steps performed under substantially fixed conditions or performed within a narrow range of conditions based on the different processing steps required for the different heat treating of workpieces.
It is further an object of the invention to provide a multi-cell thermal processing unit wherein each of the processing cells is operated at substantially fixed operating parameters.